Process for maintaining thermal conductivity of insulation in permafrost completion

ABSTRACT

This specification discloses a process of completing a well penetrating a permafrost zone of the earth whereby the lowthermal conductivity is maintained of foam insulation installed in the well. The insulation is surrounded by gas having a lower thermal conductivity than air.

United States Patent Blount [54] PROCESS FOR MAINTAINING THERMALCONDUCTIVITY OF INSULATION IN PERMAFROST COMPLETION [72] Inventor: ElmoM. Blount, Irving, Tex.

[73] Assignee: Mobil Oil Corporation [22] Filed: July 23, 1970 211 Appl.No.1 57,637

[52] U.S.Cl ..l66/244 R, 166/D1G. 1 [51] Int. Cl ..E2lb 43/00 [58]FieldofSearch ..l66/244,57,315,303,D1G. 1;

138/149, DIG. 9,113

[56] Rel'erences Cited UNITED STATES PATENTS 3,280,909 10/1966 Closmannet a1. ..166/272 X 3,380,530 4/1968 McConnell et a1... ..l 66/57 X3,397,745 8/1968 Owens et al. ..166/57 3,456,735 7/1969 McDougall etal... 166/57 UX 3,478,783 11/1969 Doyle 166/57 UX 3,511,282 5/1970Willhite et a1. ..138/113 3,525,399 8/1970 Bayless et a1 ........1 66/57 X 1 1 Feb. 15, 1972 FOREIGN PATENTS OR APPLICATIONS 134,205 9/1966u.s.s.R, ..166/D1G.1

OTHER PUBLlCATlONS Experts are Tackling the Permafrost, 0118 4 6215in't'r- I national, Vol 10, No. 8, Aug. 1970, pp 84, 89 and 90. 166-Dig.l Dupont Brochure, fRigid Urethane Foam made with DuPont Hylene Organiclsocyanates, E. l. du-

;. P ut..flsflsmaqri-Qsafiflmm sm? ELI? "952; PP-

1-7. 1387Cllular Digest World Oil, Alaskan Comple tions will beComplicated, Jan. 1970, page 85. 166-Dig. l

1 Primary ExaminerStephen J. Novosad Att0rneyWilliam J. Scherback,Frederick E. Dumoulin, William D. Jackson, Henry L. Ehrlich, Andrew L.Gaboriault and Sidney A. Johnson [57] ABSTRACT This specificationdiscloses a process of completing a well penetrating a permafrost zoneof the earth whereby the lowthermal conductivity is maintained of foaminsulation installed in the well. The insulation is surrounded by gashaving a lower thermal conductivity than air.

.. lqla mal Drawiaslfisw a PATENTEDFEB 15 I972 ELMO M. BLOUNT INVENTjORyfl E: n m M E V V 1 H H z: =52 "X24, m W

ATTORNEY PROCESS FOR MAINTAINING THERMAL CONDUCTIVITY OF INSULATION INPERMAFROST COMPLETION BACKGROUND OF THE INVENTION This invention relatesto methods for completing wells drilled through permafrost zones of theearth. More particularly, this invention relates to methods forpreventing the deterioration of the thermal conductivity of foaminsulation used in wells completed in the permafrost.

Problems have been encountered in completing wells drilled in areaswhere permafrost exists, namely, on the North Slope of Alaska andNorthern Canada. One such problem is the tendency of the permafrostaround the producing wells to melt. Melting of the permafrost wouldleave unsupported long strings of easing. Such melting would also causesubsidence of the permafrost zone in the vicinity of the well andsubsidence of the surface, thereby damaging surface installations.

Various techniques may be used in completing wells drilled throughpermafrost zones. One such technique involves the use of one or moreshear pin slip joints in the upper part of the permafrost casing string.This completion technique does not avoid melting the permafrost butrather permits the slip joint 1 in the permafrost string to telescopeupon melting of the permafrost and concurring ground, subsidence,thereby avoiding stresses in the permafrost string.

Another technique involves the use of a vacuum bottle arrangement. Twoshort lengths of concentric pipe sections are sealed to form a closedannulus and refrigeration coils are welded to the outside of the largerpipe. A vacuum is pulled on the annulus and the apparatus lowered in thewell. Refrigeration is used as needed to keep the permafrost fromthawing.

Another possible technique involves the use of insulated tubing. Forexample, the outside of the production tubing string may be wrapped withinsulation before it is run into the well.

SUMMARY OF THE INVENTION In accordance with the present invention thereare provided new and improved techniques for completing a wellpenetrating a permafrost zone of the earth wherein foam insulationformed by blowing with a gas having a lower thermal conductivity thanair is used-to insulate the produced fluids from the permafrost zone.This insulation is present in an air-filled annulus adjacent thepermafrost intermediate concentric strings of casing. The annulus issealed below the insulation. A fluid having a vapor pressure greaterthan atmospheric at permafrost temperatures and which fluid whenvaporized has a lower thermal conductivity than air is injected into theannulus. The air is removed from the annulus, leaving it filled withvapors of the fluid and the annulus is sealed at an upper location toprovide a fluidtight annulus filled with vapors of the fluid surroundingthe insulation.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a vertical section of awell penetrating the earth and illustrates an embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to amethod of completing a well drilled through the permafrost zone of theearth. A foam insulation formed by blowing with a gas having a lowerthermal conductivity than air is present in an air-filled annulusintermediate concentric strings of casing in the well and adjacent thepermafrost. The annulus is sealed below the insulation. A fluid that hasa vapor pressure greater than atmospheric at permafrost temperatures,and when vaporized has a lower thermal conductivity than air, isinjected into the annulus. Air is removed from the annulus and theannulus is filled with vapors of the fluid. Thereafter the annulus issealed at an upper location to provide a fluidtight annulus filled withvapors of the fluid surrounding the insulation.

Foams which have been blown with a gas having a lower thermalconductivity than air during their formation have been found to servevery effectively as insulation. Such a foam is polyurethane foam ineither molded or extruded form. Molded polyurethane foam is preferredhowever for several reasons. It is more convenient to install in thewells and any optimum shape and length can be obtained. Also, a highdensity, relatively impermeable skin can be obtained on the moldedsection and the individual pieces are less subject to damage whilehandling and running into the hole.

The thermal conductivity of foam fonned by blowing with a gas having alower thermal conductivity than air deteriorates somewhat with age. Ihave discovered a process whereby such deterioration is alleviated for afoam installed in an air-filled annulus in a well.

To better understand this invention a brief discussion of thermalconductivity in general and of thermal conductivity of a foam follows.The thermal conductivity (k) is defined as the amount of heattransferred per unit time between two parallel plates at differenttemperatures. The following equation l illustrates the use of k inthermal calculations:

where Q= the quantity of heat;

A the cross-sectional area;

T= the temperature of the plates or surfaces;

L= the distance between the surfaces;

At= the increment of time;

k the conductivity, and can be in any unit as required to provide thedimensional integrity of the equation. For foam the units of k aregenerally Btu-in/ft hr F.

The effective k of a foam is actually the result or sum of more than oneform of heat transfer as defined by equation (2) below:

k, the contribution to the overall conductivity due to conductionthrough the solid material;

k, the contribution to the conductivity of the gasin the cells;

k the contribution due to convection of the gas in the cells; and

k,. the contribution due to radiation.

The conduction through the solid material (k,) is insignificant. Theconvection in the vapor in the individual cells (k is influenced by thesize and shape of the cells. Reducing the size of the individual cellsreduces the convection in the vapor in the individual cells (k and alsodecreases the effect of radiation (k,) by causing more interfaces to bein the path of heat flow. The gas conductivity (k,) is apparently thepredominant parameter influencing the overall k of the foam. The foam isabout 96 percent vapor.

Polyurethane is formed by blowing with trichloromonofluoromethane(Freon-l l). The foam is formed of many individual cells having Freon-lltrapped therein and having a partial pressure something less than 1.0atmosphere. The cell walls are relatively impermeable to Freon-l l andrelatively permeable to air. The thermal conductivity (k) of Freon-l1 isabout 0.06 B.t.u.-in./ft. hr. F. while the k of air is about 0.17B.t.u.-in./ft.'*'hr. F. When foam is installed as insulation in anair-filled annulus, air diffuses into the individual cells and thusreduces the thermal conductivity of the insulation. In accordance withthis invention this reduction in thermal conductivity is alleviated byreplacing the air surrounding the foam with a gas having a lower thermalconductivity than air.

Referring to the drawing there is illustrated a particular well in whichthe invention may be practiced. Insulation 21 is shown in annulus 19intermediate production string 15 and surface string 11, and adjacentpermafrost 1. Annulus 19 is sealed below insulation 21 by, for example,a packer associated with casing hanger 14. Accumulated liquid is removedfrom annulus 19, leaving it air filled and essentially dry. Annulus 19is sealed at an upper location by casinghead 38. Fluid 40, having avapor pressure greater than atmospheric at permafrost temperatures,which fluid when vaporized has a lower thermal conductivity than air, isinjected into annulus 19. Such a fluid is dichlorodifluoromethane(Freon-12).

Fluid 40 may be injected as liquid from drum 24 via conduit 26, valve28, pipe 30, valve 32 through surface string 11 and into annulus 19where it flows to the bottom thereof. A sufficient amount of fluid 40 isinjected into annulus 19 such that when all of the liquid is vaporizedexcept an infinitesimal amount, annulus 19 is completely filled withvapors of the liquid at a pressure ofno greater than p.s.i.g.

Valve 34, associated with casinghead 38, may be used to facilitate theremoval of air from annulus 19. As fluid 40 vaporizes in annulus 19, theair in annulus 19 is displaced from annulus 19 through open valve 34.Fluid 40 has a higher molecular weight than air and thus the vaporsthereof have a higher density than air. Thus, as fluid 40 in annulus 19vaporizes, annulus 19 is filled by such vapors and air is displaced fromthe annulus through valve 34. After the annulus is essentially vaporfilled, valve 34 is closed, leaving the insulation 21 surrounded by thevapors of fluid 40.

Thepressure in annulus 19 should be maintained below the maximumallowable structure pressure of the foam insulation. The maximumallowable structure pressure is the pressure which if exceeded resultsin damage to the cell walls of the insulation. For 2 pounds per cubicfoot density polyurethane insulation this pressure is 10 p.s.i.g. at32F. and for 4 pounds per cubic foot density polyurethane insulationthis pressure is 40 p.s.i.g. at 32 F. The limiting factor ofthe maximumallowable structure pressure, 10 p.s.i.g. pressure at 32 F. (for 2lb./ft. foam) or 40 p.s.i.g. at 32 F. (for 4 lb./ft. foam), can be metby choosing a fluid which has a vapor pressure at 32 F. no greater thanthe maximum allowed pressure. Alternatively, if the fluid exhibits avapor pressure that is greater than the maximum allowable pressure, theexcess pressure may be vented at the surface by a relief valve 36 tokeep the pressure in annulus 19 below the maximum allowable pressure.The vapor pressure must be above atmospheric in order to displace theair from the annulus 19 but less than the maximum allowable structurepressure for the particular insulation used in order to prevent damageto the insulation. Thus, a fluid having these properties, and inaddition, the properties of a lower thermal conductivity than air, maybe chosen for injecting into annulus l9. Octafluorocyclobutane, FreonC-3l8 (C F cyclic), is such a fluid. Freon-l2 (CC1 F can be used if thesurface is vented through a relief valve. Mixtures of these and otherfreon compounds having the desired characteristics may be used, as wellas other gases such as ethane, propane, butane, and mixtures ofhydrocarbon gases up to pentane.

When using a fluid having a known pressure-temperature phase diagram,the temperature of the annulus may be determined by observing thepressure of annulus 19 and referring to the phase diagram. A rise intemperature as might be caused by breakdown of insulation 21 allowingannulus 19 to heat up above 32 F. can be noted directly by observing acorresponding rise in the pressure of annulus 19.

A well may be provided for carrying out this invention by drilling alarge diameter hole into permafrost zone 1 and lowering conductor pipe 3into the hole and cementing it thereto by cement 5. The hole is thenextended and a permafrost string 7 is lowered into the hole. The lowerportion of permafrost string 7 is cemented by cement 9. The hole isextended further into the earth and a surface string 11 is loweredthereinto and cemented by cement 13. Thereafter, the hole is drilled tototal depth and a production string 15 is lowered thereinto and cementedby cement 17. Production string 15 is hung in surface string 11 bycasing hanger 14. A circulating valve 16 is installed in productionstring 15 above hanger 14 and a backoff joint 18 is installed abovecirculating valve 16. After placing cement 17 intermediate productionstring 15 and surface string 11, circulating valve 16 is opened toprovide a passage between the interior of production string 1 1 andannulus l9 and liquid is used to circulate out the excess cement fromannulus 19 above hanger 14, leaving annulus l9 and production string 15liquid filled. This leaves annulus l9 and backoff joint 18 abovecirculating valve 16 free of cement. Accumulated liquid is withdrawnfrom annulus 19 adjacent the permafrost and intermediate productionstring 15 and surface string 11, leaving annulus 19 dry and air filled.This accumulated liquid may be withdrawn by swabbing the interior ofproduction string 15 while circulating valve 16 is open, therebyremoving the accumulated liquid from both the interior of productionstring 15 and annulus 19 adjacent the permafrost zone 1. Circulatingvalve 16 is then closed.

A particular method of installing insulation involves backing offproduction string 15 at backoff joint 18 and removing the productionstring 15 from the well. Insulation is then installed at the surface,about a portion of the production string removed from the well, andthereafter the insulated string is lowered into the well and reconnectedto the lower portion of production string 15. The insulation is therebyinstalled in the dry annulus. It is very important that the insulationbe installed in a dry annulus and that insulation be kept dry becauseliquid causes distortion of the insulation and greatly reduces itsinsulating value.

What is claimed is:

1. In a process of completing a well penetrating a permafrost zone ofthe earth wherein foam insulation formed by blowing with a gas having alower thermal conductivity than air is present in an air-filled annulusadjacent the permafrost intermediate concentric strings of casing insaid well, said annulus being sealed below said insulation, the stepscomprising:

a. injecting into said annulus a fluid having a vapor pressure greaterthan atmospheric at permafrost temperatures, said fluid when vaporizedhaving a lower thermal conductivity than air;

b. removing air from said annulus whereby said annulus is filled byvapors of said fluid; and

c. sealing said annulus at an upper location to provide a fluidtightannulus filled with said vapors of said fluid surrounding saidinsulation.

2. The process of claim 1 wherein said fluid has a vapor pressure ofless than the maximum allowable structure pressure at 32 F.

3. The process of claim 1 wherein said fluid has a vapor pressure ofabout 5 p.s.i.g. at 32 F.

4. The processof claim 1 wherein said fluid is octafluorocyclobutane.

5. The process of claim 1 wherein air is displaced from said annulus bysaid vapors.

6. The process of claim 1 further comprising venting said annulus tomaintain a pressure of no greater than the maximum allowable structurepressure in said annulus.

7. The process of claim 6 wherein said fluid is dichlorodifluoromethane.

2. The process of claim 1 wherein said fluid has a vapor pressure ofless than the maximum allowable structure pressure at 32* F.
 3. Theprocess of claim 1 wherein said fluid has a vapor pressure of about 5p.s.i.g. at 32* F.
 4. The process of claim 1 wherein said fluid isoctafluorocyclobutane.
 5. The process of claim 1 wherein air isdisplaced from said annulus by said vapors.
 6. The process of claim 1further comprising venting said annulus to maintain a pressure of nogreater than the maximum allowable structure pressure in said annulus.7. The process of claim 6 wherein said fluid is dichlorodifluoromethane.